diff --git a/serpent.cpp b/serpent.cpp
index 4d8cd1d0..40fab23d 100644
--- a/serpent.cpp
+++ b/serpent.cpp
@@ -8,7 +8,7 @@
 
 NAMESPACE_BEGIN(CryptoPP)
 
-void Serpent_KeySchedule(word32 *k, unsigned int rounds, const byte *userKey, unsigned int keylen)
+void Serpent_KeySchedule(word32 *k, unsigned int rounds, const byte *userKey, size_t keylen)
 {
 	FixedSizeSecBlock<word32, 8> k0;
 	GetUserKey(LITTLE_ENDIAN_ORDER, k0.begin(), 8, userKey, keylen);
diff --git a/serpentp.h b/serpentp.h
index af3fc4e7..7869a3fc 100755
--- a/serpentp.h
+++ b/serpentp.h
@@ -429,6 +429,6 @@ NAMESPACE_BEGIN(CryptoPP)
 	k[(8-r)*4 + 6] = c;		\
 	k[(8-r)*4 + 7] = d;}
 
-void Serpent_KeySchedule(word32 *k, unsigned int rounds, const byte *userKey, unsigned int keylen);
+void Serpent_KeySchedule(word32 *k, unsigned int rounds, const byte *userKey, size_t keylen);
 
 NAMESPACE_END